Synthesis and characterization of biodegradable pH-sensitive poly(acrylic acid) hydrogels crosslinked by 2-hydroxyethyl methacrylate modified poly(L-glutamic acid)

A series of poly(acrylic acid) hydrogels crosslinked by poly(L-glutamic acid)-g-(2-hydroxyl methacrylamide) (PGA-g-HEMA) were synthesized. The hydrogels showed pH-dependent swelling-deswelling behaviors. The hydrogels exhibited lower swelling ratios in an acidic medium because of the protonation of both acrylic acid (AA) and glutamic acid (GA) acid group residues. In contrast, in a basic medium (pH 8.3), the highest swelling ratios (SR) reached 5 to 10 times higher than those in the acidic medium. The SR increased sharply when the pH increased above 4, which is close to the pK(a) of PAA and PGA, resulted from the ionization of the AA and GA residues. The swelling ratios of the hydrogels with different crosslinking density in neutral medium (pH = 7) were 33.70, 30.75, 29.42, 28.11, respectively. When a model protein, BSA, was loaded within the pH-sensitive hydrogels. BSA nature was retained and protected by the hydrogels at acidic pH, and released at neutral pH. Therefore, the hydrogels may have potential applications in oral drug delivery systems. (C) 2012 Elsevier B.V. All rights reserved.